Plant growth is considerably influenced by environmental factors such as dehydration, salt, or low temperature. This is directly correlated to the issue of agricultural production, and development of crops having environmental stress tolerance has been awaited as a technique of improving agricultural production efficiency around the world. From such a point of view, development of a variety of environmental stress tolerant plants was achieved with the utilization of a classical breeding method and biotechnology such as gene recombination. In the case of plants to which enzyme genes that synthesize an amino acid (proline) or an oligosaccharide (galactinol) have been introduced, for example, proline or galactinol imparts the osmoregulatory mechanism to such plants. Accordingly, such plants gain tolerance to dehydration or salt stress. A method for imparting stress tolerance to plants via overexpression of a single gene therein is effective to some extent. However, such method is often disadvantageous for plants because it affects plant growth or because the effects attained thereby are insignificant. Many plants actually possess genes that can impart environmental stress tolerance as mentioned above, and such genes actively function to acquire tolerance when the plants receive stresses. Since these genes are considered to express upon reception of instructions from a given upstream factor, all the downstream genes are regulated and stress tolerance can be enhanced if such upstream factor can be identified. Such perspective is referred to as “regulon biotechnology” and has drawn attention in recent days. An example of the upstream factor that regulates gene expression is a “transcription factor” that directly regulates transcription of messenger RNA. Recently, DREB/CBF was identified as a transcription factor associated with dehydration and low temperature stresses. The transgenic plants thereof were found to have activated expression of downstream genes and have tolerance to dehydration, salt, or low temperature stress (Quiang Liu et al., The Plant Cell, vol. 10, 1391–1406, 1998; Mie Kasuga et al., Nature Biotechnology, vol. 17, 287–291, 1999). However, no signal transduction factor that would activate the DREB/CBF transcription factor has been found in the upstream region. Accordingly, elucidation of the upstream signal transduction factor associated with such environmental responses is considered to be useful for the development of plants having enhanced stress tolerance.
Abscisic acid (ABA) is a plant hormone associated with seed dormancy, stomatal opening and closing, and osmotic stress tolerance. ABA is deeply involved in the expression of the stress responsive gene group. When a plant receives stress such as dehydration, signal transduction takes place via the ABA-dependent path and the ABA-independent path, and this signal transduction is known to regulate the expression of the stress responsive gene group. It is reported that ABA signal transduction is involved in a wide variety of factors, and protein kinase is one of them. The present inventors have focused on the correlation between the ABA signal transduction system and protein phosphorylation, identified the SnRK2 (SNF1-related protein kinase 2) family as the protein kinase group that is specifically activated by ABA, and demonstrated that a member thereof, i.e., SRK2E, regulates stomatal opening and closing and is involved in the expression of ABA inducible genes (Riichiro Yoshida et al., Plant Cell Physiol., 43 (12), 1473–1483, 2002).